1. Field
The present invention relates to a radio communication apparatus forming a radio-communication network.
2. Description of the Related Art
In accordance with the IEEE802.11 standard related to wireless LAN, the carrier sense multiple access with collision avoidance (CSMA/CA) is employed as an access method. A wireless LAN is described below as an example employing CSMA/CA. The present invention is not limited to the wireless LAN.
In accordance with CSMA, each radio communication apparatus transmitting a frame performs beforehand the carrier sensing, and monitors the usage status of a radio channel. If a transmission of another radio communication apparatus is detected, a self radio communication apparatus postpones own transmission in order to avoid collision as much as possible. In CA, backoff control causes a time difference for each radio communication apparatus in a random fashion to avoid collision as much as possible.
FIG. 1 illustrates a communication process in accordance with the CSMA/CA technique. While a radio communication apparatus 1 transmits a transmission frame 1 as illustrated in FIG. 1, radio communication apparatuses 2 and 3 may detect a carrier of the transmission frame 1 and determines that a “busy” state is activated, and postpones the transmission of a frame. When the transmission frame 1 is completed, the radio communication apparatuses 2 and 3 wait on standby during an inter frame space (IFS), and then further wait on standby while performing carrier sensing for a backoff time within a contention window (CW). The backoff time is (slot time)×(random times). For example, the random time is a random integer generated from within uniform distribution having a range of [0, W]. For example, in the case of IEEE802.11b, CW is 31 through 1023, and the slot time is 20 μs. FIG. 2 lists the types of IFS. The methods of usage of four types of IFS are listed in FIG. 2. Distributed inter frame space (DIFS) is typically used.
If no carrier is detected within the backoff time, a frame can be transmitted. Referring to FIG. 1, no carrier is detected within the backoff time of the radio communication apparatus 2, and the radio communication apparatus 2 transmits transmission frame 2. The radio communication apparatus 3 having a backoff time prolonged by a long random time detects a carrier from the radio communication apparatus 2 during the backoff time. The radio communication apparatus 3 thus further waits on standby while performing the carrier sensing.
After the transmission of the transmission frame 2 by the radio communication apparatus 2, the radio communication apparatus 3 waits on standby during the IFS and further waits while performing the carrier sensing during the backoff time. Referring to FIG. 1, the radio communication apparatus 3 having the shortest backoff time transmits frame 3. The radio communication apparatus 1 detects a carrier from the radio communication apparatus 3 within the backoff time, and thus waits on standby while performing the carrier sensing.
In accordance with the above-described process, each radio communication apparatus fairly enjoys transmission opportunity while avoiding packet collision.
Radio communication apparatuses complying with the IEEE802.11 standard may be used as nodes to construct a wireless adhoc network as illustrated in FIG. 3. In such a wireless adhoc network, a root node STA(n+1) at the hierarchically top layer connected to a wired network is connected to nodes STA0(n), STA1(n), and STA2(n) at a second layer. The node STA0(n) is connected to node STA00(n−1), STA01(n−1), and STA02(n−1) at a third layer. The node STA1(n) is connected to a node STA10(n−1) at the third layer. The node STA2(n) is connected to nodes STA20(n−1), STA21(n−1), and STA22(n−1) at the third layer. The node STA00(n−1) is connected to a node STA000(n−2) at a fourth layer. The node STA01(n−1) is connected to a node STA010(n−2) at the fourth layer.
In such a tree-structured network, the closer to the root node the node is, the more packets the node receives from the nodes thereunder. Focusing on the nodes STA0(n), STA1(n), and STA2(n) enclosed within a broken-lined box, the STA0(n) includes six nodes including the self node as the number of nodes thereunder. The node STA2(n) includes four nodes including the self node as the number of nodes thereunder, and the node STA1(n) includes 2 nodes including the self node as the number of nodes thereuder. For example, if each node needs to transfer N packets to STA(n+1) during a period T, the node STA1(n) needs to transfer 2N packets including the packet for the self node, the node STA0(n) needs to transfer 6N packets including the packet for the self node, and the node STA2(n) needs to transfer 4N packets including the packet for the self node.
Even if amounts of data are concentrated in a localized manner, CSMA/CA provides simply uniform transmission opportunity. For example, if each node transmits 4N packets within a period T, the node STA1(n) has a redundant transmission window of 2N=4N−2N, the node STA2(n) transmits all the packets, and the node STA0(n) cannot transmit all the packets with 2N=6N−4N left. More specifically, the node STA0(n) can transmit two-thirds of the packets, and a congestion occurs.
A technique of equalizing the transmission opportunity works if all the nodes are connected to a router in a star configuration with all the nodes having equal opportunity. But the technique of equalizing the transmission opportunity is not appropriate if the wireless adhoc network illustrated in FIG. 3 is constructed.